Transparencies for the projection of light images are well known and can be formed from a transparent film base and an image or print applied thereto by an electrostatic copying process. Electrostatic copying machines are well known and generally employ sophisticated mechanisms allowing them to select imaging sheets from a stack of sheets and by the use of rollers, wheels, belts, and the like, to cause such sheets to rapidly and precisely be moved past various points in the machine during the imaging process, producing a great number of copies in a relatively short time span. Such sophisticated mechanisms include sensing devices to prevent damage to the machine if wrinkling, tearing or other deformation of the sheets occurs. For example, these sensing devices will halt operation of the machine if jamming occurs so as to avoid damage. Many of these sensing mechanisms employ photosensors which monitor the passage of the sheet through the machine. In order for such sensing mechanisms to function, the sheets need to be opaque in certain areas so as to interrupt the light beams employed in the photosensing mechanisms. Accordingly, when transparent sheet materials are used in these copying machines, they need to be rendered opaque in selected areas to operate properly in the copier.
Some copying machines require that only a small portion of the transparent sheet be rendered opaque and therefore the printing of a dark line along the top or side of such sheets is sufficient. Other machines require that a greater area of the sheet be rendered opaque or that prime image areas of the sheet be opaque. Obviously, this situation precludes the use of a transparent sheet with an unobtrusive opaque line printed along one edge.
One method of rendering a sheet opaque is by making imaging manifolds which comprise a transparent sheet and an opaque backing member generally adhered thereto by means of an adhesive. U.S. Pat. No. 3,618,752 discloses a stack of image receiving members, each being in contact with adjacent members. Each image members includes a generally rectangular, transparent, non-fibrous, flexible sheet, and a sheet of paper backing, substantially coextensive and in register with the non-fibrous transparent sheet, secured to the transparent sheet along a common leading edge or along either side edge alone or in a combination of the two. Generally, the paper sheet is adhered to the transparent sheet by a thin line of adhesive proximate a common edge of the mated sheets. The paper sheet can also be applied by other securing methods such as stapling and glueing. Optionally, the transparent sheet may be creased or scored so that when the transparent sheet has been imaged, the operator can tear the paper sheet from the transparent sheet and discard the paper sheet to leave a clean transparency.
The attachment of such paper backing sheets with an adhesive line has not provided a satisfactory imaging manifold without the use of scored or creased lines. When the paper sheet is torn from the transparent sheet, visible paper fibers usually remain on the transparent sheet in the area of the adhesive bond. When a discontinuous adhesive bond is used, for example a "dashed" adhesive line, the amount of paper fibers remaining on the transparent sheet following removal of the paper sheet is somewhat reduced. However, a more serious problem can develop in that such composite sheets have a greater tendency to jam in the feed mechanism of the copy machines.
EP0052938 Patent Specification discloses an imaging manifold comprising a transparent imaging sheet imageable in a copying machine, an opaque paper sheet underlying and in register with the transparent sheet, and an adhesive composition interposed between and bonding the paper backing sheet to the transparent sheet having peel strength less than the tear strength of the paper backing. The adhesive has a greater affinity for the surface of the paper backing than for the surface of the transparent sheet. Moreover, the adhesive is further characterized to have greater affinity for itself than for the surface of the transparent sheet. This careful balancing of adhesive properties allows the paper sheet to be peeled from the transparent sheet without leaving a visible paper residue on the transparent sheet, and without leaving any substantial adhesive residue on the transparent sheet. Further, since the sheets of the manifold are preferably joined by a continuous line of adhesive, it has less tendency to jam in the feed mechanism of the copier.
A number of adhesives are taught which provide the characteristics noted above. These include natural rubber or synthetic rubber adhesives. Other suitable synthetic polymeric adhesives include vinylacetate polymers, and ester-modified vinylacetate polymers, isooctyl acrylate/acrylamide copolymers in combination with a release agent such as a urethane based polyvinyl alcohol and octadecyl isocyanate, ethylene/vinylacetate copolymer hot-melt adhesives, and adhesives based on tacky, elastomeric, microspheres of the type disclosed in U.S. Pat. No. 3,691,140 and 4,166,152. These cited microsphere type adhesives are particularly suitable for imaging manifolds because of their noted removability from a large number of surfaces with no paper tearing.
U.S. Pat. No. 4,599,265 discloses another removable adhesive comprising a radiation cured composition comprising low levels of polar monomers, copolymerized with alkylacrylates having from about 8 to 12 carbon atoms, and crosslinked to provide removability.
The above-mentioned removable adhesives are generally either solvent based or water based and are generally not amenable to on-line processing of the entire imaging manifold because of the complicated coating and drying processes involved in manufacturing imaging manifolds. In addition, radiation cured adhesives require an inert atmosphere for curing and cannot be cured at high line speeds. In traditional off-line processes, the backing sheet is usually coated with an adhesive at a separate and different area than where the transparent sheet is made. Because the adhesive is coated at a different location, there is a need for it to be transported as a stock roll. Therefore, the backing sheet is coated with a low adhesion backsize coating to facilitate the unwinding thereof.
Another drawback with traditional radiation processes is that the monomeric adhesive compositions coated onto backings such as paper tend to bleed into the paper causing the paper to deteriorate.
Hot melt adhesives are generally less complicated to process and therefore more amenable to on-line processing. However, most known hot-melt adhesives require high processing temperatures. Therefore, the adhesive used in hot melt processing needs to be cooled prior to being combined with the transparent imaging film.
The present invention overcomes the problems discussed above with a new class of hot-melt coatable removable pressure sensitive adhesive suitable for use in an imaging manifold.